Method and apparatus for controlling neutron density



June 27, 1961 E. P. WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 19465 Sheets-Sheet 1 June 27, 1961 WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 19465 Sheets-Sheet 3 FIE-3- June 27, 1961 E. P. WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 19465 Sheets-Sheet 4 June 27, 1961 E. P. WIGNER ETAL METHOD AND APPARATUSFOR CONTROLLING NEUTRON DENSITY Filed May 14, 1946 5 Sheets-Sheet 5 QEFL1: erze FMjnreI" ale \fyoav y wflw azvzeggl:

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2,990,357 Patented June 27, 1961 sion Filed May 14, 1946, Ser. No.669,525

1 Claim. (Cl. 204-1932) The present invention relates to a device ofprimary use for the production of the transuranic element 94 by neutronsreleased during a self-sustaining nuclear chain reaction through fissionof uranium with slow neutrons. Such a device, which is usually called aneutronic'reactor is more fully described in the copending applicationof Enrico Fermi and Leo Szilard, Serial No. 568,904, filed December 19,1944, now Patent No. 2,708,- 656. Natural uranium may be used in thereaction and contains the isotopes 92 and 92 in the ratio of approximately 139 to 1. Hereinafter in the specification and the claim theterm uranium is to be understood as referring to uranium and itschemical compositions of normal isotopic content, unless otherwiseindicated by the context.

In a self-sustaining chain reaction of uranium with slow neutrons, 92 isconverted by neutron capture to the isotope 92 The latter is convertedbybeta decay to 93 and this 93 in turn is converted bybeta decay to thetransuranic element 94 By thermal neutron capture, 92 on the other hand,undergoes nuclear fission to release energy appearing as heat, gamma andbeta radiation, together with the formation of fission fragmentsappearing as radioactive isotopes of elements of lower mass numbers, andwith the release of secondary neutrons.

The secondary neutrons thus produced by the fissioning of the 92 nucleihave a high average energy, and must be slowed down to thermal energiesin order to be in condition to cause slow neutron fission in other 92nuclei. While some of the secondary neutrons are absorbed by the uraniumisotope 92 leading to the production of 94 and by other materials,enough can remain to sustain the chain reaction. u

Under these conditions, the chain reaction will supply not only theneutrons necessary for maintaining the neutronic reaction, but also willsupply the neutrons for capture by the isotope 92 leading to theproduction of 239 As 94 is a transuranic element, it can be separatedfrom the unconverted uranium by chemical methods, and as it isfissionable in a manner similar to the isotope 92 it is valuable forenriching natural uranium for use in other chain reacting systems ofsmaller overall size. The fission fragments are also valuable as sourcesof radioactivity.

The ratio of the number of secondary neutrons.pro duced by the fissionsto the original number of primary neutrons producing the fissions in achain reacting system of infinite size using specific materials iscalled the reproduction factor of the system and is denoted by thesymbol K. When K is made sufficiently greater than continue indefinitelyif not controlled at a density corresponding to a desired power output.7

To more fully understand the operation of a uranium neutronic reactor,the following brief explanation is given. During the interchange ofneutrons in a system comprising bodies of uranium of any size disposedin a slowing medium or moderator, neutrons may be lost in four ways; byabsorption in the uranium metal or compound, by absorption in theslowing down material or moderator, by absorption in impurities presentin the system, and by leakage out of the system. The neutrons which arenot lost by one of the above methods are available for fission of Uwhich produces more neutrons. In general, several neutrons are producedfor each fission and consequently suflicient neutrons are produced tomake up for the neutrons lost and those consumed by the fission of UNatural uranium, particularly by reason of its U content, has anespecially strong absorbing power for neutrons when they have beenslowed down to moderate energies. The absorption in uranium at theseenergies is termed the uranium resonance absorption or capture. It iscaused by the isotope U and does not result in fission but creates anucleus 92 which decays as previously described. It is not to beconfused with absorption or capture of neutrons by impurities, referredto later. Neutron resonance absorption in uranium may take place eitheronthe surface of the uranium bodies inwhich case the absorption is.known as surface resonance absorption, or it may take place further inthe interior of the uranium body, in which case the absorption is knownas volume resonance absorption. Volume resonance absorption is due tothe fact that some neutrons make collisions inside the uranium body andmay thus arrive at resonance energies therein. After successfullyreaching thermal energies, about 40 percent of the neutrons are alsosubject to capture by U without fission, leading to the production of 94It is possible by proper physical arrangement of the materials in themoderator to control the amount of uranium resonance absorption. By theuse of a light element such as graphite, relatively few collisions arerequired to slow the neutrons to thermal energies, thus decreasing theprobability of a neutron being at a resonance energy as it enters auranium atom. During the moderating process, however, neutrons arediifusing through the slowing medium over random paths and distances sothat the uranium is not only exposed to thermal neutrons but also toneutrons of energy varying between the energy of fission and thermalenergy. Neutrons at uranium resonance energies will, if they enteruranium atthese energies, be absorbed on the surface of a uranium bodywhatever its size, giving rise to surface absorption. Any substantialchange of overall surface of the same amount of uranium will changesurface resonance absorption. Thus, the volume ratio of moderator touranium will control resonance absorption losses of neutrons in theuranium, and this fact can be utilized to change the K factor of thereactor. The uranium may be placed in the system in the form of spaceduranium masses or bodies of substantial size, either of metal, oxide,carbide, or combinations thereof. The uranium bodies may be in the formof layers, rods or cylinders, cubes or spheres, or approximate shapes,dispersed throughout the graphite, preferably in some geometric pattern.The term geometric is used to mean any pattern or arrangement where inthe uranium bodies are distributed in the graphite with at least aroughly uniform spacing or with a roughly uniform size and shape or aresystematic in variations of size or shape to produce a volume patternconforming to a generally symmetrical system. If the pattern is arepeating or rather exactly regular one, the structure may beconveniently described as a lattice. tions are obtained when naturaluranium is used as metal spheres, but short cylinders are substantiallyequivalent.

The K factor of a mixture of fine uranium particles in graphite,assuming both of them to be theoretical-1y pure, has been calculated tobe about .785. Actual K factors as high as 1.07 have been obtained usingaggregations of natural uranium into bodies of substantial sizedispersed in moderators in various geometries, and with as purematerials as is presently possible to obtain.

The thermal neutrons are also subject to capture by the moderator. Whilecarbon has a relatively low capture cross-section for thermal neutronsan appreciable fraction of thermal neutrons (about 10 percent of theneutrons present in'the system under best conditions with graphite) islost by capture in the moderator during diffusion therethrough. Thismeans that when volume ratios are changed, the absorption in themoderator will also be changed, as the neutrons will have path lengthsvarying, before entering uranium, in accordance with the volumeratioused, and the longer time the neutrons remain in the graphite, thehigher the probability will be that they will be captured by themoderator.

All materials present in a uranium reactor except the pure uranium andthe pure moderator are classed as impurities. The materials which makeup these impuri ties all absorb neutrons in varying degrees. Since anyneutron absorbed by the impurities is lost to the chain reaction, anyvariation in the impurities present in a neutronic reactor will affectthe K factor and thus by placing more or less impurities in ditferentzones of the lattice, the K factor for each zone may be adjusted.

The effect of impurities on the optimum reproduction factor K may beconveniently evaluated to a good approximation, simply by means ofcertain constants known as danger coefficients which are assigned to thevarious elements. These danger coeflicients for the impurities are eachmultiplied by the fraction by Weight of the corresponding impurity, andthe total sum of these products gives a value known as the total dangersum. This total danger sum is subtracted from the reproduction factor Kas calculated for pure materials and for the specific geometry underconsideration. v

The danger coefficients are defined in terms of theratio of the weightof impurity per unit mass of uranium and are based on the cross sectionfor absorption ofthermal neutrons of the various elements. These valuesmay be obtained from published literature on the subject and the dangercoefficient computed by the formula:

ii a Pu A wherein Pi represents the cross section for the impurity andPu the cross section for the uranium, A the atomic weight of theimpurity and A the atomic weight for uranium. If the impurities are inthe carbon, they are computed ,as their percent of the weight of theuranium of the'syste'm. v 7

Presently known values for danger coefficients for some elements thatmay be used in a reactor are given in the following table, wherein theelements are assumed to have their natural isotopic constitution unlessotherwise indicated, and are conveniently listed according to theirchemical symbols:

Elements: Danger coeflicie nt H 10 D 0.1 He ,Be 0.04 B- 2150 C 0.012 N4.0

' ,Fe 1.5 Cd 760 Optimum condia 1 of the reactor.

reactor.

The neutronic chain reaction referred to can be made self-sustaining ina device known as a neutronic reactor wherein uranium bodies aredispersed in an eflicient neutron slowing medium or moderator, when thereactor is made to be just abovea critical size where the rate ofneutron generation inside the reactor is slightly greater than the rateof neutron loss. Under these conditions, a self-sustaining nuclear chainreaction can be obtained within the reactor having any neutron densitydesired. However, to prevent destruction of the reactor, the heat of thereaction must be controlled, and then removed by an amount providing astable temperature in the reactor at some predetermined and controlledoperating level. As the greater the number of fissions, the greater thenumber of neutrons are present to produce 92 converting to 94 isaccelerated by operating the reactor at high neutron density levels. Astable temperature in an uncooled neutronic reactor composed entirely ofmoderator and fissionable material such as, for example, graphite anduranium metal, can only be attained at a relatively low power output asthe heat generated can be dissipated only by conduction out Higher'power outputs with greater production of 94 require additional heatremoval such as by circulation of a fluid.

However, proper heat removal is complicated by the fact that in aneutronic reactor where the uranium bodies are in a lattice of uniformsize and spacing, and where the impurities are also uniformly spaced,nuclear fission and heat generation due to the chain reaction are bothgreatest at the center of the reactor and least at its edges, bothactivities following an approximate cosine curve distribution from thecenter to the edge of the reactor, as will be pointed out later. Such acentrally peaked activity limits the total power at which the reactorcan operate, to a power where the more central uranium bodies areoperating at a maximum permissible temperature. In other words, thetemperature of the uranium at the center of the reactor is a controllingfactor. If aluminum tubes and jackets are used, the maximum permissibletemperature of said aluminum jacket is 70 C., since aluminum corrodesquickly at temperatures above this. The total power output, under thesecircumstances, can therefore be only the average power developed in thereactor when the uranium at the center of the reactor has reached themaximum permissible temperature. If, however, the reactor activity curvecan be flattened across the reactor, then the central peak power canstill remain at the maximum permissible value and the total power outputof the reactor can be increased. One method of flattening this activitycurve is described in the copending application of Gale J. Young, SerialNo. 552,730, filed September 5, .1944, now Patent No. 2,774,730.

It is the principal object of our invention to so design a cooledneutronic reactor that the maximum heat generation due to nuclearfission is spread out over a large volume of the reactor so thatoperating power can be increased without excessive corrosion.

Flattening of the reactor activity curve across the reactor is alsoadvantageous in that the local heat generation is directly proportionalto the local absorption of neutrons by U In other words 94 willeventually be formed in .the uranium bodies in accordance with theneutron density to which the bodies are exposed. Flattening the reactoractivity curve across the reactor will permit a greater number ofuranium bodies to be subjected to high neutron densities.

As impurity content can control the'K factor of a reactor structure,lattices having different impurity content can provide different Kfactors in a neutronic Ordinarily, neutronic reactors have lattices inwhich uranium bodies of uniform size and shape and purity are placed inthe moderator with uniform spacing throughout, the bodies are generallyof substantially uniform size, and uniform volumes of coolant are used.

This results, in the absence of compensating factor, in a reactor havinga peaked central neutron density, and in consequence, a peaked centralheat production.

However, by using lattices having different amounts of impurities indifferent concentric zones of the reactor, and particularly bypositioning impurities to give the lowest K factor in the center zone ofthe reactor, the reactor activity curve can be appreciably flattenedacross the reactor, resulting for the same total power, in lowering therelative central peak neutron density and in raising the neutron densityin outer zones. In consequence, the activity is spread more uniformlythroughout the reactor. Cooling becomes more efficient and when thecentral uranium bodies are raised to their maximum permissible operatingtemperature, the total power output of the reactor with a flattenedactivity curve across the reactor is increased for the same centraluranium body temperature. The amount of uranium exposed to high neutrondensities is increased, and the yield of 94 is thereby increased. Byproper 'flattening the overall power of a reactor may be increased asmuch as 35 percent.

When a neutronic reactor is liquid cooled as by water, for example,jackets are used on the uranium bodies to prevent corrosion and confinefission products and then these jacketed bodies are placed insidecoolant pipes passing through the reactor. The water is then passedthrough the coolant pipes, extracting heat from the uranium bodies asthe water passes over the jackets. As heretofore explained, the jackets,the coolant pipes, and the coolant itself are all designated impuritiesand when considered as part of the lattice structure, will reduce K bythe amount of absorption caused by these added impurities.

When the size of the uranium bodies, the volume ratio of the uranium andmoderator, and the dimensions of the cooling system are the samethroughout the reactor, the K factor of all portions of the reactor willbe the same and the normally peaked central activity will result.

As increasing the amount of impurities in the central part of thereactor will reduce the K factor there, and thus provide the desiredactivity flattening across the reactor, increasing the amount ofcoolant, the thickness of coolant pipe and jacket, or both in thecentral part of the reactor will increase the impurities there, reducethe K factor there, accomplish the desired activity flattening, and, inaddition, will provide additional resistance to corrosion of theseparts.

It is, therefore, another object of our invention to flatten the neutronactivity curve across a liquid cooled reactor, and at the same timeprovide increased cooling, increased resistance to corrosion by thecoolant, or both, at central portions of the reactor.

The protective jackets on the uranium bodies perform two functions.Uranium is highly active chemically and would disintegrate rapidly ifexposed directly to flowing water. The fission process also can takeplace on the surface of the uranium bodies and highly radioactivefission fragments could thereby be injected into the coolant stream. Thejackets protect the uranium from corrosion and also prevent fissionfragments from entering the coolant stream. Since all pipes, jackets andcoolant contained in the reactor are treated as neutron absorbingimpurities, the weight of such materials is limited, in order thatreproduction factor may remain well above unity. The amount of heat thatcan be removed, therefore, depends (with pipes and jackets unchanged)upon the amount of coolant in the reactor and the rate at which it canbe circulated. Certain reactors can operate at about 500,000 kw.continuously with coolant annulus thickness of about 2 mm. for water orabout 4 mm. for diphenyl. After the point is reached where the rate ofcirculation can no longer be etnciently increased by annuli of fixedthickness, further increase in power leads to too high a temperature ofthe coolant which, has been explained, causes excessive corrosion of thealuminum pipes and jackets.

As heretofore explained, proper heat removal is complicated by the factthat in a neutronic reactor where the uranium bodies are in a lattice ofuniform size and spacing, with uniform jackets and other impurities,nuclear fission and heat generation due to the chain reaction are bothgreatest at the center of the reactor and least at its edges, bothactivities following an approximate cosine curve distribution across thereactor. However, by increasing the jacket thickness of the uraniumslugs in the center of the reactor, several advantages are obtained.First, the thicker jacket affords better resistance to corrosion at thehottest part of the reactor where such corrosion resistance is mostneeded. Second, the impurities are increased in the center of thereactor thereby flattening the activity curve. Third, it is easier tosecure watertight welds on a thick aluminum jacket than it is on a thinjacket.

Other objects and advantages of our invention may be more clearlyunderstood by reference to the following description and the attacheddrawings which illustrate, as an example, one form of reactor in whichthe invention may be used. This example of a uranium-graphite,water-cooled reactor is not to be taken as limiting, as the invention,within the scope of the appended claim, can be used in any type ofneutronic reactor wherein uranium bodies or other fissionable bodies aredisposed in a moderating medium.

In the drawings:

FIG. 1 is a diagrammatic vertical cross sectional view, partly inelevation, of the major portions of a liquid cooled neutronic reactor;

FIG. 2 is a diagrammatic vertical cross-sectional View, partly inelevation, taken on the line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary perspective view, partially incross-section of a portion of the lattice showing one of the coolanttubes containing the jacketed uranium bodies or slugs;

FIG. 4 is an enlarged fragmentary cross-sectional view of a portion ofthe lattice showing the geometrical arrangement of the uranium bodies inthe graphite moderator;

FIGS. 5, 6 and 7 are enlarged cross-sectional views of differentjacketed uranium bodies in coolant tubes showing three size slugs andjackets; and

FIG. 8 is a graph showing the relative neutron densi ties across thediameter of the reactor.

Referring to the drawings the invention will be described as embodied ina water cooled, graphite moderated uranium reactor in which the uraniumis in the form of aluminum jacketed rod segments positioned inhorizontal coolant carrying'channels in the graphite moderator.

Such a reactor embodying liquid cooling for high power outputs, up to100,000 kilowatts, for example, is shown in FIGS. 1 and 2. Specificfeatures of this reactor are more fully described, and claimed in theapplication of Edward C. Creutz et al., Serial No. 574,153, filedJanuary 23, 1945 now Patent No. 2,910,418.

The reactor proper 50 comprises a cylindrical structure built ofgraphite blocks. The reactor is surrounded with a graphite reflector 51forming an extension of the moderator and is enclosed by a fluid tightsteel casing 52, supported on I beams 54 within a concrete tank 55,erected on foundation 53. Tank 55 is preferably filled with water 56 toact as a shield for neutrons and gamma radiation.

The encased reactor is surrounded on all sides except one by the water56, and the side not surrounded, which is the charging face 57 of thereactor is provided with an inner shield tank 58 filled, for example,with lead shot and water.

Coolant tubes 59 extend through an outer shield tank 60, through theinner shield tank 58, and through the graphite moderator block 50 to anoutlet face 62" of casing 52 to empty into water 56 in tank 55. Only afew tubes 59 are shown in FIG. 1 for sake of clarity of illustration. Abacking wall 64 is placed in tank 55 spaced from outlet face 62. Coolanttubes 59 are preferably of aluminum.

On the outside of tank 55 where the coolant tubes enter the reactor, theends of coolant tubes 59 are removably capped, and are supplied withcoolant under pressure from conveniently positioned manifolds. Thuswater can be passed through tubes 59 to be discharged at outlet face 62into tank 55. Water, after having passed through the reactor is removedthrough an outlet pipe 65.

The coolant tubes 59 may be charged with aluminum jacketed uranium slugs72 (FIGS. 3-5), by uncapping the tube to be loaded and pushing slugsinto the tubes in end to end relationship. The reactor can then beloaded with sufircient uranium to make the reactor operative to producehigh neutron densities, the heat being dissipated by the coolantcirculation. This coolant may be water, for example, from a source suchas a river, passed once through the reactor, and then discarded, or, thewater may be cooled and recirculated in a closed system. If diphenyl isused a closed system is required. Loading and unloading and control ofthe reactor are described in the Creutz et a1. patent heretoforementioned.

Referring to FIG. 3, which shows the relation of a moderator coolanttube 59 and a uranium rod, it will be seen that slugs 72 forming the rodare positioned in the coolant tube 59 on longitudinal ribs'73 providinga uniform annulus of coolant around the slugs.

In this case, the jackets, the coolant itself and the pipes introduceparasitic losses which, for one specific example of a liquid cooleduranium-graphite reactor have been evaluated for a water cooled reactorcapable of continuous operation at about 100.000 kilowatts.

For such a reactor employing uranium rods disposed in graphite inaccordance with near optimum geometry conditions and utilizing uraniummetal and graphite of presently obtainable purity, the value of K wouldbe about 1.07. The value of K for the structure is determined asfollows:

K for uranium rods in graphite (including residual impurities) -1.07 Kreduction due to aluminum jackets and pipes 0.013 K reduction due tocoolant 0.023

Total K reduction for cooling system-" 0.036 0.036

The value of K for the structure 1.034

The principal dimensions of the reactor are as follows, using the Kconstant set forth above:

Axial length of active cylinder of reactor 7 meters. Radius of activecylinder of reactor 4.94 meters. Total weight of uranium metal in rods200 metric tons.

As the total K-l available for uranium-graphite reactors is only about0.1 it is obvious that the amount of coolant cannot be greatly increasedover the values given above, as the K constant would be so reduced as topreclude the construction of a reactor of practical size.

It thus follows, since the coolant and the circulating elements requiredto be placed inside the reactor are considered as parasitic impurities,that by increasing the weight of impurities in the center of the reactorover the normal weight of impurities necessary for operation, theneutron density curve may be flattened.

A portion of the moderator showing one type of lattice in which theuranium is positioned in triangular spacing is shown in FIG. 4. Thisfigure shows all tubes as standard; all having the same jacket thicknessand same coolant annulus. I

In FIG. 5, is disclosed a coolant tube 59 in which is positioned astandard slug 72. For the example given the uranium metal rod 20 is 1.7centimeters in diameter, the aluminum jacket 21 is 0.5 millimeterinthickness.

In FIGS. 6 and 7 are disclosed non-standard slugs which positioned in astandard coolant tube 59 will introduce more impurities at that positionthan would a standard slug 72. Thus, FIG. 6 discloses a slug 172 whichcontains a uranium rod 120 having a smaller diameter than the standarduranium rod 20. The outer diameter of the aluminum jacket 121 isstandard and therefore the jacket is thicker than standard. This thickerjacket'means an increase in impurities which absorb neutrons and thereduction in uranium content means that less neutrons are given oif fromfission and, therefore, neutron density is reduced for thesetwo'reasons. At the same time the increased jacket thickness increasesthe resistance to corrosion.

In FIG. 7, a second type of nonstandard slug 272 is shown. This slugcontains a uranium rod 220 that is smaller in diameter than thestandard. The aluminum jacket 22.]. is of standard thickness. Therefore,the coolant annulus is thicker than the 2.2 millimeters which has beendesignated standard. This slug 272 will also reduce the neutron densitywhen positioned in a coolant tube 59. As in the slug 172 the quantity ofuranium is less than standard and therefore the neutrons produced byfission are reduced. Furthermore, because the coolant annulus isincreased in thickness the quantity of impurities is increased sincewater is an impurity. An added benefit of this type of slug is the factthat the increased coolant annulus provides better cooling than with thestandard slug 72. Therefore, if this slug 272 is used near the center ofthe reactor increased cooling is provided at the hottest portion of thereactor. If desired, compromises of slugs 172 and 272 may be used havinga thicker jacket than standard but not as thick as 172 thus providing athicker water annulus than standard but not as thick as 272. Note thatall these non-standard slugs may be used in a standard coolant tube 59,so that the invention may be practiced on a reactor already built. It isnot necessary that the reactor be originally designed for this type ofneutron density curve flattening. For this reason ribs 222 are welded tojacket 221 so that the slug is centered in the tube 59. However, theseribs 222 may be eliminated, if non-standard tubes with larger ribs 73are used.

If the geometry of the disposition of the uranium in the moderator ofthe active portion of the reactor is uniform throughout in the loadingabove described and standard s ugs are used throughout, it follows thatthe K factor throughout the entire reactor is also uniform leading to areactor activity across the reactor having a distribution curvegenerally resembling a cosine curve as indicated by curve A in FIG. 8,which is a diagram in which the ratios of local neutron density to theaverage neutron density are plotted for different radial distances fromthe center of the reactor.

Curve A is a centrally peaked curve, indicating that when the maximumwater temperature is set, for instance at C., the total power at whichthe reactor can operate is limited by the central reactor activityrequired to bring the central slugs up to the temperature that will heatthe cooling water to this point. Proper cooling, and that thistemperature is attained at a total power output of 500 kw. are assumed.It can also be assumed that the uniform geometry used provides a Kfactor of about 1.06 throughout the reactor to give a self-sustainingchain.

reaction in the reactor at the size described, Unless the temperature ofthe central uranium is permitted to rise, assuming maximum cooling nomore power can be obtained from the reactor under these conditions.

However, the lattice in all parts of the reactor, and hence the Kfactor, does not need to be uniform, if the average K factor of thereactor is left sufficiently high that the critical size does not becometoo large or impractical. It has been found that if lattices having Kfactors that differ in the center of the reactor and in shells or zonessurrounding the center are utilized, when the reactor is assembled withthe lattice having the lowest K factor at the center, a reactor can bebuilt wherein the reactor activity is no longer peaked, but is flattenedacross the reactor as shown in curves B, and C, FIG. 8. With this typeof activity distribution, for the same total power output, neutrondensities around the central slugs and consequently, their temperatures,will fall, and the temperatures of the slugs in the intermediate zoneswill more nearly approach those in the slugs in the center of thereactor. Under these conditions, the total power output of the reactorcan then be raised until the more widespread central neutron density isthe same as when the activity follows curve A, and the central slugs areat the maximum permissible temperature again. If it is assumed thatcooling is uniform throughout the reactor, more slugs and coolant thanbefore will then be at or near maximum temperature and the total poweroutput can be greatly increased.

Thus, a density curve resembling either B or C of FIG. 8 may be attainedby loading the central coolant channels 59 with non-standard slugs suchas 172 or 272 heretofore described and loading the outer channels withstandard slugs.

If slugs with thick aluminum jackets similar to slug 172 are used in thecentral channels, the K factor of the lattice of that portion will bereduced. Furthermore, as has been explained the corrosion resistance ofthe slug is increased by the thicker jacket 172 which means that theseslugs may be safely operated at a higher temperature than standardslugs.

If slugs similar to 221 are used in the central channels of the reactor,the K factor of that portion will be reduced because of the decrease inthe quantity of uranium as compared to the standard slug and becausemore impurity is introduced in the form of the larger annulus ofcoolant. It follows that the increased amount of coolant provides bettercooling for these channels. Therefore since the limiting temperature isthe temperature of the coolant in the central channels, the power of thereactor may be raised over what it would be with the standard coolantannulus.

If a 1 percent excess K factor is available in the reactor with allslugs having standard 0.5 millimeter jackets, then by changing thecentral slugs from a di- 10 ameter of 1.7 cm. to 1.47 and increasing thejacket thickness to 3.3 mm., it is possible to increase the overallpower about 35 percent without increasing the maximum. slug temperature.The thick jacketed slugs should comprise about 26 percent of the totalnumber of slugs in the reactor.

If the same amount of metal is loaded in 1.55 cm. radius slugs with a2.5 mm. jacket, then the power may be increased about 33 percent withoutincreasing the maximum slug temperature.

The limiting factor as to the number of channels in which thenon-standard slugs may be used is the excess K factor available in thereactor. The K factor must be at least 1 or the reactor would not beself-sustaining chain reacting. Methods of calculating the average Kfactor are set forth in the Creutz et al. and Fermi et al. applicationsheretofore mentioned.

A method and means of improving the operation of a neutronic reactorhave been described. The invention may be incorporated in the design ofnew reactors, but it is useful in any reactor which is capable of beingloaded and unloaded. While a horizontally loaded, grapihte-uraniumreactor has been described the invention is usable on other types ofreactors.

Although the theory of the nuclear chain fission mechanism in uraniumset forth herein is based on the best presently known experimentalevidence, it is not desired to be bound thereby, as additionalexperimental data later discovered may modify the theory disclosed.

The above description is meant to be illustrative only and the scope ofthe invention is limited only by the appended claim.

What is claimed is:

A neutronic reactor comprising a moderator having uniformly sizedchannels parallel to one another and spaced a uniform distance apart,and composite uniformly dimensioned cylindrical bodies comprised of acore of thermal neutron fissionable material and a jacket of aluminumand positioned in each of said channels so as to be spaced from thewalls of said channels, the cores and the jackets of the bodies in thecentral channels of said reactor being respectively thinner and thickerthan the cores and jackets of the bodies in the remainder of thereactor.

References Cited in the file of this patent UNITED STATES PATENTS

